Subcommissural organ

The subcommissural organ (SCO) is one of the circumventricular organs of the brain. It is a small glandular structure that is located in the dorsocaudal region of the third ventricle, at the entrance of the cerebral aqueduct. The subcommissural organ (SCO) is one of the circumventricular organs of the brain. It is a small glandular structure that is located in the dorsocaudal region of the third ventricle, at the entrance of the cerebral aqueduct. The name of the SCO comes from its location beneath the posterior commissure, a bundle of nerve fibers interconnecting parts of the two hemispheres of the brain. The SCO is one of the first differentiated brain structures to develop. Although it is evolutionarily an ancient structure that is present throughout the chordate phylum, its arrangement varies somewhat among species. Functions of the SCO are unknown; some evidence indicates it may participate in clearance of certain compounds from the cerebrospinal fluid, and possibly in morphogenetic mechanisms, such as development of the posterior commissure. Cells of the subcommissural organ, which are specialized in the secretion of glycoproteins (see below), are arranged into two layers: a superficial layer called the ependyma and an underlying layer called the hypendyma. The ependyma consists of long, columnar cells that release their secretions into the ventricular cerebrospinal fluid. The hypendyma (which is most prominent in mammals) is characterized by numerous capillaries and glial cells. Hypendymal cells and ependymal cells both are secretory in nature; their processes project to local blood vessels and also to the subarachnoidal space. The ependymal cell bodies present a clear zonation that is especially marked in certain species: 1) In the perinuclear region, large and dilated cisternae of the rough endoplasmic reticulum (RER) are the most distinctive ultrastructural feature of virtually all species; 2) the intermediate region consists mainly of RER and Golgi apparatus; 3) the subapical region is relatively narrow, and includes microtubules, mitochondria and smooth endoplasmic reticulum; and 4) the apical region comprises a large protrusion into the ventricle. There are no neuronal cell bodies within the subcommissural organ, although the specialized cells receive some innervation from outside neurons. Ependymal cells secrete high molecular mass glycoproteins into the cerebrospinal fluid, in which the bulk of them condense to form a filamentous structure named Reissner’s fiber. The subcommissural organ/Reissner’s fiber complex is thought to be involved in the reabsorption and circulation of the cerebrospinal fluid, and with functions related to electrolyte and water balance. One of the proteins secreted by the subcommissural organ, and which is present in Reissner’s fiber, is spondin. SCO-spondin is a “giant” (5000 amino acids) glycoprotein (thrombospondin superfamily) found in Vertebrata. This glycoprotein shares molecular domains with axonal pathfinding molecules. The ependymal cells and the SCO–spondin secretion are thought to play a role in homeostasis. The ependymal cells of the SCO are also involved in the production of brain transthyretin, a protein involved in the transport of thyroid hormones in blood. Some studies indicate the presence of both tyrosine-hydroxylase-immunoreactive nerve fibers and dopamine receptors in the SCO ependyma. In addition, there is evidence suggesting that the SCO activity in adult animals may be regulated by serotonin. All capillaries in the central nervous system with a functional blood-brain barrier express glucose transporters (GLUT1). These transporters are generally absent in leaky barrier structures. The circumventricular organs that are known to have leaky barrier capillaries were stained by fibronectin antibodies but not by GLUT1 antibodies. The subcommissural organ appears to be unique in that it shows neither GLUT1 nor capillary.